With strength exercises intended to enhance the speed of movements, the resistance should be such as to let the athlete move with speed comparable to the speed in sporting actions.
The greater the amount of resistance an athlete overcomes in sporting actions, the greater the resistance used in speed exercises.
For example, weightlifters use weights of 70-80% of their 1RM where as martial artists may use weights closer to 25-40% of their 1RM.
While the strength of a muscle depends on it's cross sectional area, precise coordination of the participating motor units is required for developing maximal speed of movement.
The efficiency of the nervous system determines this coordination.
It is worth a mention here that the ability to accelerate rapidly and the ability to move at a high velocity are separate from each other.
An athlete can have a slow start but a high velocity when covering ground e.g. Usain Bolt when he broke the 100m sprint world record.
To enhance an athletes speed potential, strength exercises similar to the sports techniques should be performed so as to increase strength while refining techniques.
Strength exercises designed to improve speed in sports techniques must be similar in form, timing and rhythm to the actual sports action.
Resistance must also be comparable e.g. not too large, because too much resistance will change the movements form and will inhibit the athlete from moving as explosively as they need to.
Occasionally developing strength improves speed only at first and then has no effect. This could be because:
1) Strength has increased further than what is needed for the action.
2) The Movement time has decreased so there is not enough time to employ full strength.
3) A speed barrier has been formed. Repeating speed exercises frequently at maximal speed causes the athlete to learn that speed.
4) The coordination of movement has been altered because of the increased strength of some muscle groups.
5) Excessive resistance has changed the internal structure of the technique.
